Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed

Despite increasing recognition of storm-induced organic carbon (C) export as a significant loss from the terrestrial C balance, little is known about the biodegradation and chemical transformation of particulate organic carbon (POC) in mountainous river systems. We combined analyses of C isotopes, solution-state¹H NMR, and lipid biomarkers with biodegradable dissolved organic C (BDOC) measurements to investigate downstream changes of POC composition and biodegradability at a mountainous, mixed land-use watershed in South Korea. Water and suspended sediment (SS) samples were collected in a forested headwater stream, a downstream agricultural stream, and two downstream rivers during peak flow periods of four storm events, using either sequential grab sampling from the headwater stream to the most downstream river within a few hours around the peak flow or sediment samplers deployed during the whole storm event. DOC concentrations exhibited relatively small changes across sites, whereas POC concentrations were highest in the agricultural stream, and tapered along downstream reaches. The δ¹³C and δ¹⁵N of SS in the agricultural stream were distinct from up- and downstream signatures and similar to those for erosion source soils and lake bottom sediment, although increases in radiocarbon age indicated continuous compositional changes toward the lake.¹H NMR spectra of SS and deposited sediment exhibited downstream decreases in carbohydrates and lignin but enrichment of organic structures related to microbial proteins and plant wax. The downstream sediments had more microbial n-alkanes and lipid markers indicating anthropogenic origin such as coprostanol compared to the forest soil n-alkanes dominated by plant wax. While the BDOC concentrations of filtered waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence ratios of DOC leached from SS during a 13-day incubation were higher in downstream rivers, pointing to contribution of labile POC components to the enhanced biodegradation. Overall, inputs of microbial and anthropogenic origin, in interplay with deposition and mineralization, appear to substantially alter POC composition and biodegradability during downstream transport, raising a question on the conventional view of mountainous river systems as a passive conduit of storm pulses of POC.